1. Field of the Invention
The present invention relates to an encapsulated toner for use in the development of an electrostatic latent image formed in electrophotography, electrostatic printing and electrostatic recording, and more particularly to a charged encapsulated toner for heat-and-pressure fixing.
2. Description of the Related Art
As described in U.S. Pat. Nos. 2,297,691 and 2,357,809, conventional electrophotography comprises the steps of uniformly charging a photoconductive insulating layer, subjecting the layer to exposure, selectively dissipating the charge on the exposed portion to form an electric latent image, depositing a fine charged powder having a color called a toner to form a visual image (step of development), transferring the resultant visual image onto a transfer material, such as transfer paper (step of transfer), and permanently fixing the visual image by heating, pressure or other suitable fixation methods (step of fixation).
Thus the toner should have properties required not only in the step of development but also in the steps of transfer and fixation.
In general, a toner is subjected to mechanical frictional force derived from shear force and impact force during the mechanical action in a development apparatus and deteriorates while copying several thousands to several tens of thousands of sheets of paper. The use of a tough resin having a large molecular weight capable of withstanding the mechanical frictional force suffices for the prevention of the above-described deterioration of the toner. These resins generally have a high softening point, and thus satisfactory fixation cannot be conducted in an oven fixation process and a radiant fixation process by means of infrared radiation as the non-contact fixation system, due to its poor heat efficiency. Also, in a heat pressure fixation system utilizing a heated roller etc., which is one of the contact fixation systems which have been widely used by virtue of its good heat efficiency, it is necessary to raise the temperature of the heat roller for the purpose of attaining satisfactory fixation. This brings about an unfavorable phenomena such as deterioration of the fixation apparatus, curling of paper and an increase in the energy consumption. Further, since the pulverizability of the above-described resin is so poor, the production efficiency is remarkably lowered in the production of the toner. For this reason, a binder resin having an excessively high degree of polymerization and an excessively high softening point cannot be used.
A heat pressure fixation system utilizing a heating roller and the like is used in a wide range of applications from low speed copying to high speed copying by virtue of a very good heat efficiency because the surface of the heat roller comes into pressure contact with the surface of the toner image of the fixation sheet. However, when the surface of the heat roller comes into contact with the surface of the toner image, the toner is deposited on the surface of the heat roller and is transferred to succeeding transfer paper, that is, it tends to bring about the so-called offset phenomenon. In order to prevent the above-described phenomena, the surface of the heat roller is provided with a material having good release properties, such as a fluororesin, and further the surface of the heat roller is coated with a release agent, such as a silicone oil. The method wherein the surface of the heat roller is coated with a silicone oil and the like, however, brings about an increase in the cost due to an increase in the size of the fixation apparatus. Furthermore, this unfavorably increases the complexity of the system and is likely to bring about additional problems.
Further, as described in Japanese Patent Publication No. 493/1982 and Japanese Patent Laid-Open Nos. 44836/1975 and 37353/1982, although there is a method wherein the resin which is asymmetrized or crosslinked is used to alleviate the offset phenomenon, no improvement in the fixation temperature can be attained.
Since the lowest fixation temperature is generally between the low temperature offset disappearance temperature and the high temperature offset generation temperature, the acceptable temperature region falls between the lowest fixation temperature and the high temperature offset generation temperature. The acceptable fixation temperature can be lowered and the acceptable temperature region can be broadened by making the lower fixation temperature as low as possible and making the high temperature offset generation temperature as high as possible, which contributes to a saving of energy, fixation at a high speed and prevention of the curling of the paper.
For this reason, a toner always exhibiting good fixation and offset resistance has been desired in the art.
A proposal has heretofore been made as to the use of an encapsulated toner comprising a core material and an outer shell which covers the surface of the core material for the purpose of improving fixation at a low temperature.
When a plastic deformable low melting point wax or the like is used as the core material (see U.S. Pat. No. 3,269,626, Japanese Patent Publication Nos. 15876/1971 and 9880/1969 and Japanese Patent Laid-Open Nos. 75032/1973 and 75033/1973), although fixation can be attained by the application of pressure alone, the fixation strength is so poor that this method can be used only for limited applications.
When use is made of a liquid core material, fixation can be attained by the application of pressure alone in the case where the strength of the shell material is small. In this case, however, the shell is frequently broken within the developing device which causes the inside of the device to become stained. On the other hand, when the strength of the shell material is excessively large, high pressure is necessary for breaking the capsule, which brings about the formation of an image having an excessively high gloss. This made it difficult to regulate the strength of the shell material.
For this reason, for use in heat pressure fixation, a proposal has been made on a microcapsulated toner for heat roller fixation, comprising a core material made of a resin having a low glass transition point which is capable of improving the fixation strength, although single use thereof brings about blocking at a high temperature, and an outer shell comprised of a high melting point resin wall formed by interfacial polymerization for the purpose of imparting blocking resistance (see Japanese Patent Laid-Open No. 56352/1986). In this toner, however, since the wall material (or the outer shell) has a high melting point, the performance of the core material cannot be sufficiently attained.
According to the same line of thinking, a proposal has been made on an encapsulated toner for heat roller fixation which is improved in the fixation strength of the core material (Japanese Patent Laid-Open Nos. 128357/1988, 128358/1988, 128359/1988, 128360/1988, 128361/1988 and 128362/1988). However, since these toners are produced by spray drying, a burden is imposed on the production facilities. Furthermore, since no device or contrivance is conducted concerning the outer shell, the performance of the core material cannot be sufficiently attained.
Further, as proposals of an encapsulated toner comprising a core material and an outer shell, U.S. patent application Ser. No. 680,538, European Patent Publication No. 0,453,857 which was published Oct. 30, 1991, U.S. patent application Ser. No. 833,502 are cited.
Although it has been tried to control the chargeability of the encapsulated toner by incorporating a charge control agent into the shell of the encapsulated toner or at the surface thereof, problems were created in that the charge control agent might be removed from the toner by, for example, friction with the carrier and adherence to the carrier to reduce the quantity of the electrification or charge of the toner, causing scumming or stain, or scattering the toner in the apparatus during the development. Further, when no charge control agent is present on the toner surface, the charging velocity is reduced depending on the kind of carrier and the scumming and scattering of the toner are often caused in the high-speed printing process.
An object of the present invention is to provide an encapsulated toner for heat-and-pressure fixing which exhibits excellent offset resistance and blocking resistance in a heat pressure fixation system utilizing a heated roller, and which can be fixed at a low fixing temperature.
Another object of the present invention is to provide an encapsulated toner for heat-and-pressure fixing wherein the electrification or charging properties of the toner can be controlled from inside the encapsulated toner to stably and repeatedly form a scumming-free, clear image.
The present invention relates to an encapsulated toner for heat-and-pressure fixing which is comprised of a heat-fusible core containing at least a coloring material, a binder and an electric charge control agent selected from the group consisting of a positive type electric charge control agent, a negative type electric charge control agent and a mixture of a positive type electric charge control agent and a negative type electric charge control agent at a weight ratio of one to the other ranging from 1:0 (exclusive) to 1:0.5, and a shell formed so as to cover the surface of the core, wherein the main component of the shell is a resin prepared by reacting (A) an iso(thio)cyanate compound comprising
(1) 0 to 30 mole % of monovalent isocyanate and/or isothiocyanate compounds, and
(2) 100 to 70 mole % of at least divalent isocyanate and/or isothiocyanate compounds,
with (B) an active hydrogen compound comprising
(3) 0 to 30 mole % of a compound having one active hydrogen atom reactive with isocyanate and/or isothiocyanate groups, and
(4) 100 to 70 mole % of a compound having at least two active hydrogen atoms reactive with isocyanate and/or isothiocyanate groups,
at a molar ratio of component (A) to component (B) of between 1:1 and 1:20, and wherein at least 30% of all of the linkages in which an isocyanate and/or isothiocyanate group participates are thermally dissociable linkages.
Namely, the present invention includes an encapsulated toner for heat and pressure fixing which comprises a meltable core containing at least a colorant and one or more charge control agent(s) of positive charging type and a shell covering the core surface, characterized in that the shell comprises, as a main component, a resin obtained by reacting the following compounds:
(1) 0 to 30 molar %, based on the total of isocyanate compounds and/or isothiocyanate compounds, of a monovalent isocyanate compound and/or isothiocyanate compound, and
(2) 100 to 70 molar %, based on the total of isocyanate compounds and/or isothiocyanate compounds, of a divalent or higher isocyanate compound and/or isothiocyanate compound,
with the following compounds:
(3) 0 to 30 molar %, based on the total of compounds reactive with an isocyanate group and/or isothiocyanate group, of a compound having one active hydrogen atom reactive with an isocyanate group and/or isothiocyanate group, and
(4) 100 to 70 molar %, based on the total of compounds reactive with an isocyanate group and/or isothiocyanate group, of a compound having two or more active hydrogen atoms reactive with
an isocyanate group and/or isothiocyanate group, in such amounts that the molar ratio of (1) plus (2) to (3) plus (4) is in the range of 1:1 to 1:20, and at least 30% of all the bonds in which the isocyanate groups and/or isothiocyanate groups of the resin participate are thermally dissociable bonds.
The present invention also includes an encapsulated toner for heat and pressure fixing which comprises a meltable core containing at least a colorant and one or more charge control agent(s) of negative charging type and a shell covering the core surface, characterized in that the shell comprises, as a main component, a resin obtained by reacting the following compounds:
(1) 0 to 30 molar %, based on the total of isocyanate compounds and/or isothiocyanate compounds, of a monovalent isocyanate compound and/or isothiocyanate compound, and
(2) 100 to 70 molar %, based on the total of isocyanate compounds and/or isothiocyanate compounds, of a divalent or higher isocyanate compound and/or isothiocyanate compound,
with the following compounds:
(3) 0 to 30 molar %, based on the total of compounds reactive with an isocyanate group and/or isothiocyanate group, of a compound having one active hydrogen atom reactive with an isocyanate group and/or isothiocyanate group, and
(4) 100 to 70 molar %, based on the total of compounds reactive with an isocyanate group and/or isothiocyanate group, of a compound having two or more active hydrogen atoms reactive with an isocyanate group and/or isothiocyanate group,
in such amounts that the molar ratio of (1) plus (2) to (3) plus (4) is in the range of 1:1 to 1:20, and at least 30% of all the bonds in which the isocyanate groups and/or isothiocyanate groups of the resin participate are thermally dissociable bonds.
The thermally dissociable linkages are preferably those formed by the reaction of a phenolic hydroxyl and/or thio group with an isocyanate and/or isothiocyanate group.
The characteristic feature of the present invention is further remarkable when the main ingredient of the heat-fusible core in the encapsulated toner is a thermoplastic resin with a glass transition point of 10.degree. to 50.degree. C. as the binder, and the softening point of the encapsulated toner is 80.degree. to 150.degree. C.
The present invention further relates to a toner composition for heat-and-pressure fixing which comprises the above-described encapsulated toner and a fine powder of a hydrophobic silica.
Further scope and applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.